The ultrasound imaging system is widely used in the medical field for displaying an image of a target object such as a human body. Ultrasound signals are transmitted to the target object and then reflected from the target object, thereby forming the ultrasound image.
In order to transmit the ultrasound signals, the ultrasound imaging system generally includes a transducer array, which includes a plurality of transducers and a pulser for driving each transducer. Each transducer generates ultrasound signals in response to the pulse applied from the pulser. During transmission of the ultrasound signal, a timing point of the ultrasound generation at each transducer is controlled, thereby transmit-focusing the ultrasound signals at a predetermined position within the target object. In other words, the pulser pulses the respective transducers with different time delays so that the ultrasound signals reach a desired position within the target object at the same time.
The ultrasound signals reflected from the target object are received by the transducer array. The time for the reflected signals to reach the respective transducers is different depending on the location of the transducers. Therefore, in order to compensate for the time difference among the transducers, a beamformer applies and adds the delayed time, with respect to the reflected signals, which are received by the respective transducers, and generates receive-focused signals.
The B-mode scanning technique, which provides two-dimensional (2-D) sectional images, is a very basic technique among various imaging techniques. Resolution, contrast and frame rate are critical factors in determining the quality of the ultrasound image. Among these factors, resolution is the most important index. It can be decomposed as follows: lateral resolution, the direction orthogonal to the direction of the traveling beam (i.e., the direction of the scan line); axial resolution, the direction in which the beam propagates; and elevational resolution, the direction orthogonal to the plane of the other two directions.
Since the late 1980s, electronic scanning techniques, wherein ultrasound signals are received by transducer arrays and focused in accordance with signal processing, have been used to improve resolution. FIG. 1 illustrates the conventional production of 2-D images using linear transducer arrays. When transducer array 1 is used, the lateral resolution at the focal points may be enhanced by increasing the number of channels. However, the resolution at locations other than the focal points is degraded.
Dynamic receive-focusing addressed this problem to make it possible to obtain high-resolution images at any viewing position. This technique is a significant improvement over the convention production of 2-D ultrasound images. However, this method can only provide optimum resolution in the vicinity of a fixed transmit focal depth. The resolution outside the vicinity of the focal depth is still degraded.
Multiple zone transmit-focusing overcomes this problem of resolution degradation by forming an image from the combination of images obtained from multiple transmission/reception processes for multiple focal points (depending on the positions of the scan lines according to their distances). An image is divided into several zones along the scan depth and ultrasound signals are transmitted with respect to every scan line as many time as the number of zones. FIGS. 2A to 2C illustrate a conventional multiple zone transmit-focusing technique where an image is divided into two zones. Referring to FIG. 2A, a transducer array transmits ultrasound signals to be focused at a first focal point Transmission Focal Point 1, thereby forming an image of a first zone Zone 1. Referring to FIG. 2B, the transducer array transmits ultrasound signals to be focused at a second focal point Transmission Focal Point 2, thereby forming an image of a second zone Zone 2. Referring to FIG. 2C, a final image is formed by combining the images of the first focal point Transmission Focal Point 1 and the second focal point Transmission Focal Point 2. Consequently, the entire beam pattern as well as the lateral resolution is greatly improved. However, since the frame rate decreases with the number of transmission/reception processes and ultrasound signals must be transmitted to each scan line as many times as the number of the focal points, this method causes a degradation in the frame rate.
Bi-directional dynamic focusing, a synthetic aperture technique, is another solution to the problem of resolution degradation. In order to obtain a single scan line, several transmission/reception processes are required. This delays the time for obtaining data and frequently introduces a phase distortion due to movement of the target object. Furthermore, when a single unit device is used to transmit ultrasound signals, the signal to noise ratio (SNR) is low.
Conventional B-mode ultrasound images of human soft tissue exhibit a speckle pattern, which shows up as grains in the image. The speckle pattern is an interference phenomenon, resulting from a plurality of minute scatterings of the transmission signal as it passes through the surrounding medium. The speckle pattern is different from the random noise distribution of a system. The speckle pattern coincides with the scan lines. Thus, the speckle patterns are not reduced by averaging the scan lines.
In order to reduce speckle patterns, conventional ultrasound imaging methods employ a frequency compounding method wherein the signal values obtained by repeated transmissions of ultrasound pulses having different center frequencies are averaged. Referring to FIG. 3, speckle patterns are reduced by obtaining and averaging the transmission value of a first pulse signal 1st having a first center frequency f1 and a second pulse signal 2nd having a second center frequency f2. However, this method also reduces the frame rate due to the number of transmission/reception processes.
FIGS. 4A and 4B illustrate a method for obtaining scan lines in a conventional ultrasound imaging system. While the conventional ultrasound imaging system forms a scan line with a single ultrasound transmission/reception process, FIGS. 4A and 4B illustrate three scan lines formed by three ultrasound transmission/reception processes. Referring to FIGS. 4A and 4B, the number of scan lines in a conventional system increases as the number of ultrasound transmission/reception processes increases, which leads to frame rate degradation.
Referring to FIG. 5A, in a conventional one-dimensional (1-D) transducer array, a mechanical lens is employed for resolving the elevational direction. Resolution quality of locations out of the focal point of the lens is remarkably low due to the fixed-focusing in both directions of transmission and reception. If another transducer array is added for resolution in the lateral direction, the transducer array becomes 2-D, which may raise the resolution quality but complicates the system.
Referring to FIG. 5B, if the system employs a 1.5-D transducer array having smaller transducer elements in the elevational direction, the system can alter transmission focal points, thereby improving the image resolution, but perform the dynamic receive-focusing with greatly simplified hardware. However, the elevational resolution of locations out of the fixed-focused regions is still slightly degraded.